// add it into the HttpServletRequest object
req.setAttribute("hello", hello);

// create a dispatch from the ServletContext object
// to do the redirection for me =)
// don't forget to specify where you wanna go.
RequestDispatcher rd = getServletContext().getRequestDispatcher(target);

// now here is the magic... you are passing the
// HttpServletRequest obj and HttpServletResponse obj
// to the jsp page
rd.forward(req, res);

.....
// code here will not be execute since you have
// pass the control to "a.jsp"
}
....

In the "a.jsp" page, you can retrieve the "hello" object
by doing this:
<%
String hello = (String)request.getAttribute("hello");
out.println(hello);
%>

It's very helpful by doing this because you can hide
all the business logic inside the Servlet and the JSP
file will just be doing printing...